The invention relates to Radio Frequency Identification (RFID) systems and, particularly, to an RFID system designed to continuously track articles and personnel as they move through buildings.
RFID products typically have three components: (1) a tag (the item being identified), (2) an interrogator (a device which detects the presence of a tag), and (3) a system (typically including cabling, computers, and software which tie together the tags and interrogators into a useful solution). RFID products are typically designed to detect tags when they pass within range of a few fixed or handheld interrogators.
RFID systems are usually deployed as high-end replacement technology for bar coding. RFID and related systems include passive RFID systems, active RFID systems, infrared ID systems, and Electronic Article Surveillance (EAS) systems.
The tags in a passive RFID system do not carry on-board power. The interrogator in such systems transmits operating power for the tags. Such systems generally have a detection range of a meter or less, although somewhat longer ranges have been achieved. Typically, these systems operate in the 125-kilohertz radio band.
Most passive RFID systems work as follows. An interrogator emits an electromagnetic field for the purpose of powering the tag. A coil in the tag is powered by the electromagnetic field, causing the tag""s circuitry to xe2x80x9cwake up.xe2x80x9d The tag uses this power to send an identifying signal back to the interrogator.
Although most passive RFID systems are read-only (that is, the tags in such system respond to a query by reading information from their memory and sending the information back to the interrogator), the tags used in some passive RFID systems have a limited ability to accept information and instructions from the interrogator, for example read/write capabilities in smart cards (electronic money) and xe2x80x9celectronic manifestsxe2x80x9d in industrial applications.
Passive RFID tags have been employed in conjunction with access control, smart cards, vehicle identification (AVI), waste management, item tracking, animal identification, manufacturing control, materials handling, and a variety of other purposes.
One fundamental design goal of any RFID system is for the weak signal emitted from the tag to be distinguishable from the much stronger signal emitted by the interrogator. Some strategies for doing this include:
Frequency shifting. Circuitry in the tag receives a carrier from the interrogator, translates the signal to another frequency, and emits a response modulated onto that second frequency.
Half duplex operation. The tag is charged by the interrogator. When the interrogator""s charging circuit turns off, the tag uses the stored power to respond.
Modulated backscatter. The tag modulates its antenna cross section to identify itself to the interrogator.
Delayed retransmission. Surface acoustic wave (SAW) devices retransmit the interrogator""s carrier after a delay.
The tag""s identity is indicated by time variations in the delayed response.
Active RFID systems require battery-powered tags. The battery permits a longer detection range of between 3 and 100 meters. These systems are capable of locating tags with higher accuracy than passive RFID systems and typically operate in the 400, 900, or 2440 megahertz bands. Active tags tend to enable multiple tags to be within range of an interrogator by the use of xe2x80x9chandshakingxe2x80x9d between the tags and interrogator, so that each tag transmits its signal in turn. Communication between tag and interrogator in active RFID systems is also typically faster than with passive tags.
Most active RFID tags respond to the interrogator when polled, in accordance with a communications protocol. Some active RFID tags xe2x80x9cchirpxe2x80x9d (transmit) a signal spontaneously at predetermined intervals. A tag""s chirped signal is detected by the interrogator if the tag is in range of the interrogator.
Infrared systems (IRID), while not RFID systems, also endeavor to detect and identify the position of mobile tags. A typical IRID system includes a tag that chirps its identity at randomized intervals. Infrared readers located in the ceiling detect these transmissions, and report them to a host. The transmission rate from the tag to reader is typically about 600 baud. Motion detectors in the tags enable the tags to transmit more frequently when in motion. The tags are typically about the size of dominos.
EAS systems are intended to deter theft in retail environments. EAS tags are fairly unreliable, very low in cost, and limited in capabilities. Although they track mobile tags, they are not generally considered to be RFID products, because EAS tags are uncoded and cannot be distinguished from one another.
A system for tracking mobile tags includes cell controllers with multiple antenna modules which generate a carrier signal received by the tags. The tags respond by transmitting identification codes at randomized intervals, the codes being modulated onto the carrier signal. The antenna modules located, for example, in the ceiling, receive the responses and send them to a cell controller, which processes them and uses them to calculate tag locations by proximity and triangulation techniques. The distance of a tag from a particular antenna module is calculated by measuring the round trip signal time. The cell controllers send processed data derived from the received signals to a host computer. The host computer collects the data and resolves them into positional estimates. The host computer archives data in a data warehouse, such as an SQL Server.
Among the advantages of the invention are one or more of the following.
One advantage of the invention is that it is designed to remain in constant communication with the tags while covering a complete facility. The system is able to identify and calculate the location of tags even in the presence of severe multipath effects.
Another advantage of the invention is that it uses tags with low power consumption requirements, allowing the lifetime a powered tag to approximate the lifetime of the tag battery itself. Furthermore, tags can enter a low-power mode when not in use, thereby further conserving power.
Another advantage of the invention is that it is scaleable. A small number of widely-spaced antenna modules can be used to roughly locate tags within a facility. If a more accurate tag location is desired, additional antenna modules can easily be added to the system. Furthermore, new tags can be added to the system without requiring any system re-configuration.
Another advantage of the invention is that it mitigates problems caused by the collision of tag signals. Because tags spontaneously wake up and xe2x80x9cchirpxe2x80x9d on a randomized basis, multiple tags are unlikely to transmit signals simultaneously. Furthermore, in some circumstances the system is able to predict when tag signal collisions will occur and respond accordingly.
Another advantage of the invention is that tags can respond to multiple cell controllers simultaneously.
Other features and advantages of the invention will become apparent from the following description and from the claims.